The field of the invention is neurological stimulators and, more particularly, implantable neurostimulators for use in Functional Electro Stimulation (FES) of skeletal muscles.
A great number of persons suffer from neurological disorders which render otherwise functional muscles usable due to the inability to naturally innervate the afflicted muscle groups. The most common forms of this malady are paraplegia and quadraplegia in which the normal nerve path to the muscle is interrupted, usually by spinal injury.
In such cases of paralysis, not only is the patient immobilized, but additionally, the muscles atrophy through time due to the lack of exercise. It is known, however, that artificial stimulation can be applied to skeletal muscles to prevent the deleterious effects of inactivity. Even further, research has been conducted, and is continuing, in the field of Functional Electro Stimulation (FES) in which the limited use of paralyzed members may be partially restored.
Many techniques are known for artificially stimulating nerves. The most common approach in FES is to use an implanted device which has one or more wires connected to the nerves to be stimulated. An external control unit transmits a coded Radio Frequency (RF) signal to the implant. The RF signal is coded with information to command the stimulation by the implant, for example, by supplying values for intensity, duration, and when to apply the stimulation.
The implant derives power from the RF signal and decodes it to extract the stimulation parameters. Batteries are therefore not needed and, in any event, would not be practical for this type of application since FES requires much more electrical power than, for example, a heart pacemaker which only triggers a self-sustaining nerve impulse.
In the simplest prior systems of this type, two wires are routed from the implant to each nerve to be innervated. At the attachment point on the nerve, a bare end of each wire is sutured onto the nerve. A current is then forced through the nerve between the attachment points of the two wires, thus stimulating the nerve.
It has recently been discovered that this type of "single-point" stimulation results in rapid fatigue of the stimulated muscle. The cause of the fatigue is that the nerve contains many separate areas of sensitivity which are responsible for simulating certain dedicated muscle fibers. By using single-point stimulation, the same muscle fibers are repeatedly stimulated and thus fatigue rapidly. It has further been discovered that when normal, undamaged nerves are stimulated naturally (e.g. by the brain), the stimulation is not confined to a single portion of the nerve, but rather "rotates" throughout the nerve during a single contraction. This "rotation" of the natural stimulation results in a variation of the muscle fibers being activated so that fatigue of a single muscle group is prevented.
As a result of this research, a system has been developed, as described in Austrian Pat. No. 330342, to replicate the above described natural rotation of stimulation by using several electrodes attached around the circumference of a nerve. The electrodes can then be energized in various combinations of polarity to produce many different zones of stimulation in the nerve. By cycling the electrode polarities, the stimulation is made to "rotate" in a more natural manner, preventing premature muscle fatigue
This approach, however, requires approximately five electrodes per nerve, all of which must be connected to the implanted device. As a result, only a few nerves can be connected in this manner before the number of connections needed at the implanted device becomes prohibitive.
In an FES implant, it is desirable to stimulate a large number of muscles in order to replicate as nearly as possible natural movements. For example, to produce a natural gait, it is desirable to stimulate as many as 8 muscle groups in each leg. Prior implants were not able to accommodate the large number of connections that are needed for multiple nerve FES systems, especially when up to five electrodes are needed for each rotating stimulation electrode.
Further, prior implant cases were typically sealed units, which are unrepairable. Due to chemical effects to which the implant is subjected, splicing, etc. is not an acceptable practice for long term reliability. If a fault develops in one of the multiple leads or the implant electronics, the entire system must be replaced. This, of course, involves considerable surgery on the patient.
For similar reasons, the prior practice of suturing the electrodes onto the nerve has serious drawbacks. First, it is not possible to form a good seal between the wire and its surrounding insulation on the lead. This compromises the long term reliability of the lead. Secondly, the suture is a frail connection which may tear away from the nerve. That danger is complicated if rotating stimulation is to be used because each of the multiple electrodes would have to be sutured to the nerve in close proximity.